This invention relates to a method of adaptive forward and/or inverse quantization and an electric circuit for use in carrying out the method.
In general, such a method is used to quantize each of sampled values derived from an input signal into a quantized code of a predetermined number of bits and to adaptively vary a quantization step or step size in accordance with a level which the sampled value has. With the adaptive method, it is possible to reduce the number of bits necessary for quantization of each sampled value, as compared with a nonadaptive method and to precisely reproduce or inversely quantize the quantized code of the reduced number of bits.
As will later be described with reference to one of about ten figures of the accompanying drawing, a conventional adaptive quantization method is disclosed in an article contributed by N. S. Jayant to The Bell System Technical Journal, Vol. 52, No. 7 (September 1973), pages 1119-1144, under the title of "Adaptive Quantization with a One-Word Memory." According to the Jayant method, the sampled value is adaptively quantized on a transmitting side by linearly varying the step sizes in accordance with an increase of a level of the sampled value. The sampled value may take a negative, a zero, or a positive value each of which may be represented by a preselected number of bits. It is possible with the Jayant method to precisely and faithfully carry out forward and inverse quantization of the sampled value even when the input signal widely varies. However, this method is disadvantageous in that the quantized code can not be produced on a receiving side if the step sizes are not initially coincident with each other on the transmitting and the receiving sides. In addition, the method is susceptible to a transmission error which might occur in a transmission line between the transmitting and the receiving sides.
Alternatively, another conventional method is disclosed in a correspondence by Goodman et al to IEEE Transactions on Communications, November 1975, pages 1362-1365 under the title of "A Robust Adaptive Quantizer." With the Goodman et al quantizer, the step sizes are nonlinearly varied in order to alleviate an influence of noncoincidence between the initial step sizes of the transmitting and the receiving sides and to reduce the transmission error. However, a reduction of quantization precision is inevitable when the input signal has a high or a low level as compared with an average level of the input signal.